1. Field of the Invention
The present invention relates to a method of and apparatus for positioning beads around a tire component material wound around the building drum of a green tire building machine and locking the beads in position.
2. Description of Related Art
FIG. 6 shows a conventional machine for building green tires. The tire building machine comprises a headstock 55, a rotary shaft 56 supported at one end by the headstock 55 and a building drum 57 mounted on the rotary shaft 56. The periphery of the axial midportion of the drum 57 is provided with a building bladder 58 which is freely inflatable and contractable. The periphery of each of opposite ends of the drum 57 has a side bladder 59 which is also freely inflatable and contractable.
FIG. 7 is an enlarged view in section of the portion VII shown in FIG. 6, i.e., a conventional bead locking apparatus. The bead locking apparatus is disposed inside the side bladder 59 and positioned at each of opposite end portions of the drum 57. The apparatus has bean support segments 60 in an annular arrangement. These segments 60 are movable radially of the drum by being guided by slide keys 61 which are arranged radially. An annular recess 62 is formed in the outer periphery of the annular arrangement of bead support segments 60. An annular elastic member 63 of rubber or the like is fitted in the recess 62. The outer periphery of the elastic member 63 is smaller in diameter than large-diameter portions 60a of the annular assembly of segments 60 which portions define the recess 62 therebetween. The rotary shaft 56 is provided with a drive member 64 which is movable axially of the shaft and is connected to each segment 60 by a link 65. The drive member 64, when moved axially, moves the segments 60 radially.
Green tires are built by the following process using the abovementioned building machine.
A plurality of tire component materials 50, for example, a rubber layer 51 as the innermost layer, a steel cord reinforcement layer 52 over the layer 51, and a carcass 53 over the layer 52 and the layer 51 are each wound around the building drum 57 with the bead support segments 60 and the side bladders 59 contracted and a portion of each side bladder 59 one over another. Each of beads 54 is then brought in over convolution of the tire component material 50 to position around the segments 60. Next, the drive member 64 moves axially thereby expanding the annular arrangement of segments 60 to radially push the material 50 outward with the large-diameter portions 60a of the segment assembly, with the bead 54 positioned between the large-diameter portions 60a, 60a and pressed by the elastic member 63. The bead 54 thus positioned between the portions 60a is thereby prevented from moving in the axial direction and locked. The side bladder 59 is then inflated. The outer end portion of the material 50 is turned up around the bead 54 by forcing the inflated side bladder 59 inward axially of the building drum 57. Subsequently, the building bladder 58 on the drum 57 is inflated, whereby the material 50 positioned between the pair of beads 54 is shaped to a toroidal form.
The steps formed by the large-diameter portions 60a of the bead support segments 60 and the elastic member 63 of smaller diameter prevent the bead 54 from deflecting in the axial direction.
The steel cord reinforcement layer 52 of the tire component material 50 has a small width only sufficient to surround the bead 54 and is positioned only at each end of the carcass ply 53 as seen in FIG. 5. The steel cords 52a are in a bias arrangement as shown in FIG. 8.
Further as shown in FIG. 10, the carcass ply 53 of the tire component material 50 includes steel cords 53a which are arranged in parallel to the radial direction.
The bead locking method employed in the conventional building process described has the following drawbacks.
When the annular arrangement of bead segments 60 is radially expanded, the large-diameter portions 60a of the segments 60 first cause the side bladder 59 to press the tire component material 50. Accordingly, the material 50 radially expands to increase its circumferential length. This results in a delay in causing the elastic member 63 to contact the material 50 through the side bladder 59. The steel cord reinforcement layer 52 is therefore so deformed that the bias angle .theta. of the steel cords 52a ; becomes small as shown in FIG. 9. Further the steel cords 53a of the carcass ply 53 are bent by contact with the reinforcement layer 52 as shown in FIG. 11. Since such deformation is uneven, there arises the problem that the cord path (distance between the beads at opposite ends) becomes uneven over the entire circumference to impair tire uniformity. Further if a tire is made with steel cords 53a of the carcass ply 53 in a bent state, another problem is encountered, that is, when the tire is inflated with air, the steel cords 53a stretch and rotate as bent to impair the durability of the tire.
Further when the arrangement of segments 60 is expanded, a clearance occurs circumferentially of the arrangement between the adjacent segments, with the result that with respect to the circumferential direction, the material 50 is locally pressed by the large-diameter portions 60a of the segment arrangement and left unpressed also locally. Thus, the material 50 is deformed unevenly with respect to the circumferential direction of the tire.
Stated more specifically, the material 50 is pressed radially from inside by the pair of large-diameter portions 60a on opposite sides of the bead 54 and is also pressed radially from outside by the bead 54. Accordingly, the material 50 is supported at three points, and this mode of support is low in stability. When the segment arrangement is further expanded, the material 50 is held between the bead 54 and the elastic member 63. When thus held, the material 50 is supported with good stability with respect to the axial direction.
Until the elastic member 63 starts to press the tire forming material 50 after the large-diameter portions 60a start to press the tire component material 50, the material 50 is supported by these portions 60a and the bead 54 with very low stability. Accordingly, it is likely that slippage will occur between the material 50 and the large-diameter portions 60a or between the bead 54 and the material 50, although such slippage does not always occur. As a result, the material 50 as positioned between the bead 54 and the large-diameter portions 60a is stretched by being pulled or will remain unstretched to undergo uneven deformation. It is also likely that the bead 54 will move in the axial direction.
If the material 50 deforms unevenly with respect to the axial or circumferential direction, or if the bead 54 becomes shifted, the cord path (the bead-to-bead distance) will vary circumferentially of the tire. This causes the problem that variations in cord path adversely affect tire uniformity.